VFR 750 Regulator / Rectifier Cooling

As reports of dead rectifier/regulator units for VFR 750s were on the increase, and I have a '97 VFR 750, I decided to have a good look at my unit and do some testing.

I am an electronics engineer by trade, although for the past 10 years I have been more involved in designing computer networks and moved away from component/circuit design.  I live in Canberra, Australia.

Anecdotal evidence suggested that the rectifier/regulator was running very hot, and the frame around the unit was also very hot although it was unsure if it was the frame that was heating up the rectifier/regulator or vice versa.

Preliminary

First I took the bike for an hour's ride through some twisty country roads and light traffic suburbia, then I measured some temperatures with a digital thermometer.

All temperatures are Degrees Celsius.  For the members of the list using Degrees F, to convert from C to F,

F = 1.8 x C + 32

e.g. 69.8C = 157.64F.   Ouch!!!

Results

• Ambient temp. : 26.5 C
• Rectifier temp. : 69.8 C  (can you believe that, no wonder it burns the F@#$ out of your finger!)
• Frame temp (top rail of sub frame above rectifier) : 58.3 C (the temperature of the frame rails drops rapidly the further away from the rectifier/regulator that measurements are taken so I believe it is the rectifier/regulator that is shedding heat into the frame).
• Right side rear cowling temp. : 40.2 (outside wall)

Compare

Observations

1. Electronic components are usually damaged by three things: excessive heat, vibration and voltage spikes.
2. The heat of the rectifier/regulator unit is quite high, high enough to reduce the life of the unit.  If this temp is lowered it should extend the life of the unit (vibration / voltage spikes aside) IMHO.
3. The rectifier/regulator does not mate flush with the frame (slight air gaps) and is not transferring heat to the frame as efficiently as it could (hence the temp. differential between rectifier/regulator and the frame rail).
4. There is about 3.5 inches of clearance between the rectifier/regulator and the cowling on the 94+ VFR which should allow room for additional heat sinks to be fitted.
5. 1986-89 VFRs have a substantial heat sink included as standard, 1990 models onwards do not.  (This information was derived from the Haynes workshop manual).

Rectification Attempt 1

• Applied heat transfer compound between rectifier/regulator and frame to remove air gaps and improve heat transfer.

Results

• Ambient temp. : 26.8 C
• Rectifier temp. : 64.5 C  (better)
• Frame temp. (top rail of sub frame above rectifier) : 62.8 C  (closer to the rectifier/regulator temp.)
• Right side rear cowling temp. : 40.4  (outside wall).

Compare

Rectification Attempt 2

Components

1. Heat transfer compound
2. Passive heat sink (no fan).  This was a heat sink normally used for power transistors and is 1.75 inches square with "fingers" 2 inches high, made of aluminium.

Procedure

• Applied heat transfer compound between rectifier/regulator and frame to remove air gaps and improve heat transfer.
• Installed passive heat sink to top of rectifier/regulator with silicon rubber sealant.

Results

• Ambient temp. : 27.5 C
• Rectifier temp. : 55.6 C  (better again)
• Frame temp. (top rail of sub frame above rectifier) : 53.9 C
• Right side rear cowling temp. : 46.4  (getting hotter).

Compare

Rectification Attempt 3

Components

1. Heat sink assembly.  I decided to borrow one of the lists' suggestions and organised a Pentium PC heat sink and fan assembly (for Pentium 200).
2. Heat transfer compound.
3. Silicon rubber sealant.

Procedure

1. The bottom of the rectifier/regulator has a metal plate, which is used to transfer heat to the frame.  I covered this with enough heat transfer compound to fill in the air gaps between the rectifier/regulator and the frame and bolted the unit back onto the frame.  
2. The top of the rectifier/regulator is covered in silicon rubber to protect the components.  I fixed the heat sink assembly to this with silicon rubber sealant.  The heat sink / fan assembly is large enough to sit on the metal frame of the rectifier/regulator.  Apply the silicon rubber sealant to the centre of the heat sink only, so as not to block the cooling fins around the rim of the heat sink.  (A dab of epoxy resin can be applied to the corners of the heat sink assembly where it meets the rectifier/regulator for added adhesion).    
3. Power to the heat sink fan was obtained from the brake light connector (in Australia, our headlights are wired to be on when the ignition is turned on.  We don't get a choice).  Power for the fan can be taken from any number of points that are switched by the ignition, as the fan only draws .08 amps.  The earth was taken from the bolt which fastens the rectifier/regulator to the frame.    

* The heat sink assembly is only 1 inch high and allows ample clearance to the rear cowling.

Results

Another 1 hour ride (yeah!!!) on the same roads as before, although the day had heated up from earlier.

• Ambient temp. : 29.3 C
• Rectifier temp. : 44.63C  (yes!)
• Frame temp. : 42.5 C
• Right side rear cowling temp. : 29.5  (seems safe).

Compare

After all this I went out for another ride for about 2 hours with friends.  When I got back I measured everything again with almost identical results.

Tentative Conclusion

• The temp of the rectifier/regulator is about 28 degrees cooler with the heat transfer compound between the rectifier/regulator and the frame, and the heat sink and fan installed.  That's got to be a good thing.
• The rear cowling is actually a little cooler now.
• This doesn't actually prove a thing, but I am personally more confident that the rectifier/regulator will last longer.
• I can't believe that the unit gets that hot as standard!

If anyone has questions / reservations or concerns please raise them.  There are plenty of inventive minds on the list, and we should be able to work them out.

Good luck!

Andrew,

Canberra, Australia.

asai@jna.com.au andysai@msn.com caradoc@geocities.com

Edwin New's home page (no other VFR content at present)

Update 13 December 1997 :

• Temperatures of regulator/rectifier consistent with previous measurements.
• Right rear cowling still remaining cool and shows no signs of heat fatigue.

Appendix : Summary of temperature readings

Summary of temperature readings

	Standard	Heat Transfer Compound (HTC)	HTC & Passive Heat Sink	HTC & Heat Sink with Fan	Difference from Standard
Ambient temp.	26.5 C	26.8 C	27.5 C	29.3 C	n/a
Rectifier temp.	69.8 C	64.6 C	55.6 C	44.3 C	-25.5 C / -45.9 F
Frame temp.	58.3 C	62.8 C	53.9 C	42.5 C	-15.8 C / -28.4 F
Cowling temp.	40.2 C	40.4 C	46.4 C	29.5 C	-10.7 C / -19.2 F

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